Recently a new type of semiconductor laser, designated "quantum cascade" or "QC" laser, was discovered. See, for instance, U.S. Pat. Nos. 5,457,709 and 5,509,025, and U.S. patent application Ser. No. 08/825,286, titled "Article Comprising An Electric Field-Tunable Semiconductor Laser", filed Mar. 27, 1997 by Capasso et al., all incorporated herein by reference. Furthermore, U.S. Pat. No. 5,745,516, filed Nov. 6, 1996 by the instant inventors, is closely related to the instant application.
Known QC lasers are unipolar semiconductor lasers that have a core region that comprises a multiplicity of essentially identical repeat units, each repeat unit comprising one or more quantum wells (QWs). Successive carrier transitions from a higher to a lower energy state result in photon emissions, with the photon energy depending on the structural and compositional details of the repeat units. Thus, QC lasers can be designed to emit radiation at substantially any desired wavelength in a rather wide spectral region. Specifically, they can be designed to emit in the mid-infrared (mid-IR), a spectral region for which there are few convenient radiation sources.
Although several types of QC lasers are known, it would clearly be desirable if further types were known, since this would give designers increased design freedom, and possibly provide improved characteristics. This application discloses a new type of QC laser.